Wireless transmission of power is very useful in, for example, charging of battery operated devices. One of the more popular applications for this technology is charging cell phones. The cell phone (receiver) is placed on a power transmitter such that tuned coils in the receiver align with tuned coils in the transmitter. The aligned coils act as a transformer to wirelessly transfer power from the transmitter to the receiver. The energy received by the phone is used to charge the battery in the phone.
One of the most challenging aspects to wireless power transfer is the prevention of heating of metal objects (foreign objects) in the magnetic field (i.e., between the coils of the transmitter and receiver). The Wireless Power Consortium (WPC) has been trying to address this problem for several years and is steadily making progress. To date, the primary method used to detect when a foreign object is present is “Power Loss Accounting” in which Equation (1) accounts for all power terms:Pinput=Preceived+Pknown losses+Punexpected losses  [1]Where: Pinput is the input power supplied to the transmitter from an external source for transmission,
Preceived is the total power received by the device being charged including expected losses in the receiver,
Pknown_losses is the known losses in the power transmitter, and
Punexpected_losses is the unexpected losses assumed to be due to a foreign object.
When Punexpected_losses exceeds a threshold, the transmitter can shut down to prevent potentially dangerous heating. This can occur when a foreign object is incorrectly located in the magnetic field, or is between the transmitter and the receiver. The threshold for several common objects such as coins, rings, and foil candy wrappers that prevents exceeding thermal targets has experimentally been determined to be about 500 mW. However, several measurement limitations, as explained further below, make it difficult to accurately determine when the loss is caused by a foreign object or when the loss is caused by other acceptable losses within the transmission system.